The proliferation of digitized media has created a pressing need for copyright protection and enforcement schemes. One of such schemes is to embed copyright information into the digital materials to be protected. Such embedded signaling systems should meet some basic requirements. First of all, the embedded signal should be unobtrusive, i.e., its presence should not interfere with the work being protected. In the case of embedding data into audio signals, this means that the embedded signal should be inaudible. That is, the composite signal (the audio and the embedded data) should not be readily distinguishable from the original audio signal by listening. Secondly, the embedded data should be robust, and any attempt to remove or obscure the data would result in noticeable signal quality degradation.
Among various techniques which have been applied for embedded signaling, the spread spectrum communications theory based technique has received a great amount of attention because of its superior resistance to a variety of attacks, intention or unintentional signal distortions which include data compression, low-pass and high-pass filtering, re-sampling, requantization and digital-to-analog and analog-to-digital conversions. In spread spectrum communications, a narrow-band signal is transmitted using a much larger bandwidth such that the signal energy present in any single frequency is imperceptible. In an embedded signaling system, an audio signal is viewed as a communication channel, and the embedded data is viewed as a signal that is transmitted through it. Therefore, the embedded signal decoder behaves like the receiver in a communications system. It is well known that in all spread spectrum systems, code synchronization is necessary because the code is the key to despreading the desired information and to spreading any undesired signals. The coded data embedded in an audio signal must be accurate in both its code pattern position in time and its rate of chip generation with respect to the decoder's reference code. In practice, however, this required time accuracy is not always available, especially when the data-embedded audio signal has gone through some of the common signal processing/distortion processes mentioned above.
An embedded signaling system is described by Robert D. Preuss, et al., in U.S. Pat. No. 5,319,735 entitled “Embedded Signaling.” This patent is incorporated herein by reference.